Lift Requirement and Parameter Accessors up to GenericSignature

Start treating the null {Can}GenericSignature as a regular signature
with no requirements and no parameters. This not only makes for a much
safer abstraction, but allows us to simplify a lot of the clients of
GenericSignature that would previously have to check for null before
using the abstraction.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -646,7 +646,7 @@ public:
         },
         [&out] { out << ", "; });
     out << " where "
-        << nominal->getGenericSignature()->getGenericParams().front()->getName()
+        << nominal->getGenericSignature().getGenericParams().front()->getName()
         << " : " << DummyProtocolName << " {}\n";
     return true;
   }